1. Field of the Invention
The present invention relates to a cutting insert adapted to cut the inner wall surface of a prepared hole in a workpiece and a cutting tool on which the cutting insert is installed.
2. Description of the Related Art
There have been proposals for cutting inserts used for what is called boring to cut the inner wall surface of a prepared hole in a workpiece to form a machined hole and cutting tools with such cutting inserts installed thereon. In connection with such cutting inserts and tools, a technique for reducing the roughness of the inner wall surface of the machined hole is disclosed in Japanese Patent Laid-Open No. 2007-283467.
The cutting insert described in Japanese Patent Laid-Open No. 2007-283467 is characterized by being externally shaped like a polygonal flat plate and including two polygonal surfaces and a side surface lying orthogonally to the two polygonal surfaces, and in that cutting edges are provided at respective crossing ridge portions between each of the polygonal surfaces and the side surface, corner rounded portions are formed in respective corners of the polygonal surface, and between each of the corner rounded portions and at least one of a pair of the cutting edges intersecting at the corner portion, a linear edge inclined more inward than the one cutting edge is formed in a plan view of the polygonal surface.
Furthermore, the cutting tool described in Japanese Patent Laid-Open No. 2007-283467 includes the cutting insert removably installed thereon and adapted to cut an internal wall surface of a prepared hole in a workpiece using the cutting edges of the cutting insert, thus forming a machined hole, and is characterized by including a tool body that is rotated around an axis, a recess formed in an outer periphery of a leading end portion of the tool body, and a mounting seat formed backward of the recess in a tool rotating direction, and in that the cutting insert is installed in such a manner that one of the polygonal surfaces of the cutting insert is directed forward in the tool rotating direction, that one of the corner portions projects outward in a radial direction of the tool body and toward the leading end of the tool body, and that the linear edge is directed outward in the radial direction of the tool body.
The following are described concerning the cutting insert described in Japanese Patent Laid-Open No. 2007-283467. In the polygonal surface of the cutting insert, a breaker groove recessed in the polygonal surface and extending along marginal ridge portions of the polygonal surface and a rib portion partitioning the breaker groove and extending from a central portion of the polygonal surface toward each of the marginal ridge portions are formed. The breaker groove includes an inclined surface formed therein and inclined in such a manner that the inclined surface retracts gradually from a part of the polygonal surface in which the breaker groove is formed as the distance from the corner portion increases as seen in a plan view of the side surface, and at the crossing ridge portion between the inclined surface and the side surface the cutting edge is formed. Thus, even when the cutting insert is installed such that a relief angle is provided for its surface directed outward in the radial direction of the tool body so as to serve as a peripheral relief, the radial rake angle of a major cutting edge directed toward the leading end of the tool body can be set to a positive side. This allows the major cutting edge to cut better to significantly reduce cutting resistance, thus preventing the tool body from vibrating to allow cutting to be accurately achieved. The cutting insert can also be prevented from being damaged. Furthermore, since the major cutting edge cuts better, a machined surface resulting from cutting has more appropriate surface roughness.
However, in the cutting insert described in Japanese Patent Laid-Open No. 2007-283467, at the linear edge directed outward in the radial direction of the tool body, the angle (wedge angle) between the side surface (relief face) and the inclined surface (rake face) formed in the breaker groove decreases, thus significantly reducing cutting edge strength of the linear edge. Hence, the linear edge may be chipped or become defective early, precluding the surface roughness of the inner wall surface of the machined hole from being kept in an appropriate condition for a long time.